Integrated circuit with mems element and manufacturing method thereof

ABSTRACT

An integrated circuit comprising a MEMS (microelectromechanical system) element in a plane of the integrated circuit, the MEMS element being suspended in a cavity over a substrate, said cavity including a first cavity region in said plane spatially separating an edge of the MEMS element from a wall section, said edge being arranged to be displaced relative to the wall section; and a second cavity region in said plane forming part of a fluid path further including the first cavity region, said fluid path defining a first volume; and a third cavity region in said plane defining a second volume in fluid connection with the second cavity region, wherein the maximum width of the second cavity region is larger than the maximum width of the third cavity region, the second and third cavity regions having maximum widths that are larger than the maximum width of the first cavity region.

FIELD OF THE INVENTION

The present invention relates to an integrated circuit (IC) comprising aMEMS (micro-electromechanical system) element suspended in a cavity overa substrate and an actuator for said MEMS element, wherein said cavitycomprises a first region spatially separating the MEMS element from theactuator by a first width.

The present invention further relates to a method of manufacturing suchan IC.

BACKGROUND OF THE INVENTION

Surface micro-machining is a technique whereby freestanding and moveablestructures are made on top of a substrate using thin film deposition andetching techniques. In this way, both the MEMS element, e.g. aresonator, oscillator, capacitor element, pressure sensitive element andso on, and its package can be processed on top of a substrate such as asilicon wafer. The MEMS element typically has a height of only severalthin films measuring about 10 μm in total thickness. Furthermore,surface micro-machining allows for the definition of many thousands ofMEMS elements onto a single wafer without making use of expensiveassembly steps. This makes MEMS technology a particularly promisingtechnology for miniaturization of functionality on an IC.

The MEMS element may be responsive to an actuator in the IC, e.g. incase of a capacitive MEMS, in which case the measured response of theMEMS element to the actuation signal is translated into a value of aparameter of interest. The MEMS element, which is typically suspended ina cavity over the substrate of the IC, is spatially separated from theactuator by a gap filled with a gas or vacuum, which gap forms a part ofthe cavity volume. In order to maximize the sensitivity of the MEMSelement to the actuation signal, or to maximize the sensitivity of theIC to the detection of the displacement of the MEMS element, this gapshould be kept as narrow as possible, as the electrostatic force appliedby the actuator on the MEMS element typically scales with 1/W², whereinW is the width of the gap expressed as the distance from the edge of theMEMS element facing the actuator or sensor and the edge of the actuatoror sensor facing the MEMS element. For this reason, this gap often formsthe narrowest clearance of the MEMS element from the cavity walls.

Such narrow clearances can cause problems in the manufacture of suchMEMS elements. Many manufacturing processes rely on one or more wetetching steps to release the MEMS element, i.e. to form the cavityaround the MEMS element, which can cause the MEMS element to stickagainst the cavity walls during subsequent drying steps. This phenomenonis commonly referred to as stiction, and renders the MEMS devicenon-functional. This problem can be addressed by critical point dryingsteps, but such techniques are relatively immature and costly.

Moreover, even if stiction problems can be avoided, it has been found bythe present inventors that filament formation between the MEMS elementand the cavity wall can still occur. An example of such filamentformation is shown in FIG. 1, which depicts a scanning electronmicroscope image of a MEMS element 10 cleared from its surroundings bynarrow trench 20 (the actuation gap) and a wider trench 30 providingelectrical insulation from its surroundings. In the amplified section ofthe actuation gap, the formation of filaments bridging the gap can beclearly identified, as further indicated by the arrow.

SUMMARY OF THE INVENTION

The present invention seeks to provide an IC comprising a MEMS elementin a cavity that is less prone to such filament formation.

The present invention further seeks to provide a method of manufacturingan IC comprising a MEMS element in a cavity that is less likely to leadto filament formation in the actuation gap.

According to an aspect of the present invention, there is provided anintegrated circuit comprising a MEMS (microelectromechanical system)element (10) in a plane of the integrated circuit, the MEMS elementbeing suspended in a cavity over a substrate, said plane including afirst cavity region spatially separating an edge of the MEMS elementfrom a wall section of the cavity, said edge being arranged to bedisplaced relative to the wall section; and a second cavity regionforming part of a fluid path further including the first cavity region,said fluid path defining a first volume; and a third cavity regiondefining a second volume in fluid connection with the second cavityregion, wherein the maximum width of the second cavity portion is largerthan the maximum width of the third cavity regions, the second and thirdcavity regions having maximum widths that are larger than the maximumwidth of the first cavity region, and wherein at least a part of thesecond volume is excluded from the fluid path.

The present invention is based on the insight that the filamentformation predominantly tends to occur in the narrowest parts of thecavity, i.e. in the narrowest trenches, which indicates that thefilament formation is likely to be caused by residues left behind in thecavity when drying the cavity following the cleaning of the cavity withorganic solvents such as isopropyl alcohol (IPA). The capillary forcesin the narrowest parts of the cavity will cause these parts to resistdrying, such that these parts tend to be the last parts of the cavity todry, which can cause residue accumulation and filament formation.

The present invention is based on the further insight that the provisionof a wide second cavity region will trigger the nucleation of a dryingfront in this region in the plane of e.g. the active material layer,which drying front will progress from the second cavity region to othercavity regions in the plane, thereby forming an increasing void, i.e. aregion free of liquid. As part of the third cavity region is separatedfrom the fluid path including the first cavity region and the secondcavity region, this void will eventually separate the liquid in thethird cavity region from the liquid in the first cavity region, suchthat contaminants in the third cavity region no longer can reach thefirst cavity region as the diffusion medium, i.e. the liquid connectingboth regions, no longer is present. Consequently, the final contaminantconcentration in the first cavity region is reduced, thus reducingfilament formation in the first cavity region.

It is noted that instead, filament formation in the third cavity regionmay occur. However, as long as filament formation in the third cavityregion does not significantly affect MEMS performance, e.g. because thethird cavity region is located away from the MEMS element or is locatedalong an substantially stationary edge of the MEMS element, this isperfectly acceptable.

In order to prevent a significant portion of the contaminants in thecleaning liquid from reaching the first cavity region, the second volumepreferably is larger than the first volume. For the same reason, themajority of the second volume preferably is excluded from the fluidpath. More preferably, the whole of the second volume is excluded fromthe fluid path.

In an embodiment, the first cavity region extends from the second cavityregion, and the third cavity region is fluidly connected to the secondcavity region via a channel portion having a width not exceeding thewidth of the first cavity portion. This ensures that the void formationprogresses in a direction away from the channel portion, such that thecontaminants in the third cavity region are excluded from the firstcavity region almost immediately after the void formation in the secondcavity region, thus minimizing the amount of contaminants that candiffuse from the third cavity region to the first cavity region once thedrying process has started.

In an embodiment, the plane further includes a plurality of channelsextending from the first cavity region into the cavity wall section,each of said channels having a channel width that does not exceed thewidth of the first cavity region. The narrow channels act as sinks forthe residues, such that the risk of filament formation in the actuationgap is further reduced.

In accordance with another aspect of the present invention, there isprovided an integrated circuit comprising a MEMS (microelectromechanicalsystem) element formed in a material layer over a substrate andsuspended in a cavity; a capping layer over the cavity, said cappinglayer comprising a plugged vent hole, wherein the cavity comprises afirst cavity region spatially separating an edge of the MEMS elementfrom a wall section of the cavity, said edge being arranged to bedisplaced relative to the wall section; a first further region inbetween the MEMS element and the substrate; and a second further regionlocated in part in between the MEMS element and the capping layer, saidsecond further region comprising a first section in between the MEMSelement and a part of the capping layer including the plugged vent hole;and a second section in between an unreleased portion of the materiallayer and a further part of the capping layer not including a pluggedvent hole, wherein the first section is separated from the secondsection by a channel portion having a width not exceeding the maximumwidth of the first cavity region.

In this alternative embodiment, a contaminant reservoir, i.e. the secondsection, with a preferably large volume is provided over an unreleasedportion of the active material layer which is connected to the rest ofthe cavity and the one or more vent holes via a narrow channel. Thelocation of the vent hole in connection with the first section willtrigger void formation in the first section in the initial stages of thedrying process, whereas the capillary forces in the narrow channel willprevent the second section from drying at the same time. Hence, the voidformation will separate the contaminated liquid in the second sectionfrom the first channel region, such that the overall contaminantconcentration in the first channel region at the end of the dryingprocess will have been reduced, thus reducing the risk of filamentformation in this region.

In accordance with another aspect of the present invention, there isprovided a method of manufacturing an integrated circuit comprising aMEMS element suspended in a cavity, the method comprising providing afirst sacrificial material layer on a substrate; providing an activematerial layer on the first sacrificial material layer; patterning theactive material layer to form the MEMS element, a first cavity regionspatially separating an edge of the MEMS element from a wall section ofthe cavity, said edge being arranged to be displaced relative to thewall section; a second cavity region forming part of a fluid pathfurther including the first cavity region, said fluid path defining afirst volume; and a third cavity region defining a second volume influid connection with the second cavity region, wherein the maximumwidth of the second cavity portion is larger than the maximum width ofthe third cavity regions, the second and third cavity regions havingmaximum widths that are larger than the maximum width of the firstcavity region, and wherein at least a part of the second volume isexcluded from the fluid path; forming a part of the cavity by partiallyremoving the first sacrificial material layer through the patternedactive material layer using a first etch recipe; forming a secondsacrificial material layer over the patterned active material layer;forming a capping layer over the second sacrificial material layer, saidcapping layer comprises at least one opening; and completing said cavityby removing the second sacrificial material layer through the at leastone opening using a wet etch recipe.

The IC of the present invention can be manufactured in a straightforwardmanner by adjusting the patterning of the active material layer to formthe channel portions in the trenches surrounding the MEMS portion.Hence, no additional processing steps are required to improve thereliability of the MEMS of the IC by the reduction of the risk offilament formation in e.g. the actuation gap.

In an alternative aspect of the present invention, there is provided amethod of manufacturing an integrated circuit comprising a MEMS elementsuspended in a cavity, the method comprising providing a firstsacrificial material layer on a substrate; providing an active materiallayer on the first sacrificial material layer; patterning the activematerial layer to form the MEMS element, a first cavity region spatiallyseparating an edge of the MEMS element from a wall section of thecavity, said edge being arranged to be displaced relative to the wallsection; forming a part of the cavity by partially removing the firstsacrificial material layer through the patterned active material layerusing a first etch recipe; forming a second sacrificial material layerover the patterned active material layer; patterning the secondsacrificial material layer to define a first section on the MEMSelement, a second section on an unreleased portion of the material layerand a channel portion connecting the first section to the secondsection, the channel portion having a thickness not exceeding themaximum width of the first cavity region; forming a capping layer overthe second sacrificial material layer, said capping layer comprises atleast one opening over the first section; and completing said cavity byremoving the second sacrificial material layer through the at least oneopening using a wet etch recipe.

The IC of the present invention can be manufactured in a straightforwardmanner by adjusting the patterning of the second sacrificial materiallayer to form a contaminants reservoir over the active material layer.In this case, a single additional processing step suffices to improvethe reliability of the MEMS of the IC by the reduction of the risk offilament formation in e.g. the actuation gap.

In an embodiment, in said methods the patterning step further comprisesdefining a plurality of channels extending from the first cavity regioninto said wall section, each of said channels having a channel widththat does not exceed the maximum width of the first cavity region. Thisfurther reduces the risk of filament formation in the actuation gapwithout requiring additional processing steps to release the MEMSelement.

The methods may further comprise cleaning the cavity by rinsing thecavity with a solvent such as isopropyl alcohol; and subsequently dryingthe cavity to remove etch residues from the cavity. Due to the presenceof the channel portions, the risk of filament formation in the actuationgap during said drying step is significantly reduced. The cleaning stepmay further comprise rinsing the cavity with water prior to the dryingstep.

BRIEF DESCRIPTION OF THE EMBODIMENTS

Embodiments of the invention are described in more detail and by way ofnon-limiting examples with reference to the accompanying drawings,wherein:

FIG. 1 shows a SEM image of an aspect of a MEMS element suffering fromfilament formation;

FIG. 2 shows a SEM image of an aspect of an IC comprising a MEMSelement;

FIG. 3 schematically depicts a manufacturing method of an IC comprisinga MEMS element;

FIG. 4 schematically depicts a drying sequence of a prior art ICcomprising a MEMS element;

FIG. 5 schematically depicts a drying sequence of an IC comprising aMEMS element according to an embodiment of the present invention;

FIG. 6 schematically depicts a non-limiting example design aspect of anIC comprising a MEMS element according to an embodiment of the presentinvention;

FIG. 7 schematically depicts a drying sequence of an IC comprising aMEMS element according to another embodiment of the present invention;

FIG. 8 schematically shows an aspect of an IC according to anotherembodiment of the present invention;

FIG. 9 schematically depicts a drying sequence of an IC comprising aMEMS element according to yet another embodiment of the presentinvention;

FIG. 10 depicts the simulation results of a final contamination level inthe actuator gap as a function of the number of channels extending froma first cavity region such as an actuator gap; and

FIG. 11 schematically shows a design aspect of an IC according to anembodiment of the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

It should be understood that the Figures are merely schematic and arenot drawn to scale. It should also be understood that the same referencenumerals are used throughout the Figures to indicate the same or similarparts.

In the context of the present invention, a cavity region is a portion ofthe cavity in a particular segment or plane of the cavity. For instance,a cavity region in the plane of an active material layer is a regionthat has is located in between the upper plane and lower plane of anactive material layer in which a MEMS element is defined. A cavityregion is delimited by an active material layer wall portion, a junctionwith another cavity region or a combination of both. A cavity region isdelimited from another cavity region by the presence of a non-linearjunction between the two cavity regions or a channel that connects thetwo cavity regions. A channel is a conduit between two adjacent cavityregions that is narrow enough to exhibit non-negligible capillary forcesin the presence of a liquid in the channel. The width of a channel issignificantly less than the width of any of the two cavity regions, e.g.no more than 20% or 10% of the minimum width of the cavity regions.

Where reference is made to the width of a cavity region, this width isdefined as the distance between opposing delimiting edges of the cavityportion. The width of a cavity region does not exceed the length of acavity region.

In the present invention, any void that is fluidly connected to thecavity is considered to form part of the cavity. This for instanceincludes trenches etched into a section of the active material layerthat is not released from the underlying substrate, e.g. because thesacrificial material separating this section from the substrate is notremoved.

A non-limiting example of a MEMS device is shown in FIG. 2, which showsa top view of a dog bone resonator MEMS element 10 released from thelayer in which it is formed by trenches 20 and 30. The trench 20separates the MEMS element 10 from a portion 12 of the IC, whereas thetrench 30 separates the MEMS element 10 from a portion 14 of the IC. Thetrenches 20 and 30 allow the release of a sacrificial materialunderneath the MEMS element 10, thereby releasing the MEMS element 10from the underlying substrate.

The MEMS element 10 is typically suspended in the thus formed cavity byone or more anchoring structures. In FIG. 2, two anchoring structures 16and 18 are shown by way of non-limiting example. Such anchoringstructures may play an active or passive role in the operation of theMEMS element 10. In case of a passive role, the anchoring structures aresimply anchoring the MEMS element in its cavity. In case of an activerole, e.g. as is the case in the dog bone resonator MEMS element 10 ofFIG. 2, a signal, e.g. a (direct) current I₀ may be applied to the MEMSelement 10 via anchoring input structure 16, with one or more (gate)electrodes 12 applying a gate voltage V_(g0) to the dog bone resonatorMEMS element 10 across the actuation trench 20, which becomes modulatedwith the resonance frequency of the resonator such that the anchoringoutput 18 can be read out in a piezoresistive manner, given that theoutput voltage can be expressed as V_(out)=I₀(R+ΔR), in which ΔR is thevariable resistance component caused by the in-plane oscillation of thedog bone resonator MEMS element 10.

The operating principle of such a resonator is explained in more detailin an on-line publication “56 MHz Piezoresistive MicromechanicalOscillator” By J. J. M. Bontemps et al., which was retrieved from theInternet athttp://www.memsland.nl/publications/publications%202009/Bontemp_Transducers2009.pdfon Thursday 19 Jul. 2012.

It should however be understood that the present invention is notlimited to a particular type of MEMS element 10, as this type has nosignificant bearing on the principles of the present invention. Forinstance, whereas in FIG. 2 a device is shown exhibiting an in-planeoscillation mode, it is equally feasible to have a device exhibitingout-of-plane oscillation behaviour. Equally, although in FIG. 2 thesensing output 18 and actuator 14 are configured in a perpendicularorientation, it is equally feasible to adopt a linear arrangement ofthese elements. The MEMS does not have to be a resonator, but may be anysuitable type of MEMS. Other obvious variations will be immediatelyapparent to the skilled person.

Various design considerations play a role in choosing the dimensions ofthe cavity regions, e.g. trenches 20 and 30 for releasing the MEMSelement 10. From a manufacturability perspective, it may for instance bedesirable that the trenches 20 and 30 have the same width, as thisprovides good control over the etch process for releasing the MEMSelement 10. On the other hand, in case of a side of the MEMS elementfacing a region 12 of the IC for interacting with the MEMS element 10,e.g. a gate electrode or sensing element for measuring or invoking thedisplacement of the MEMS element 10 relative to the edge or wall of theregion 12, the trench 20 should be kept as narrow as possible, aspreviously explained.

At the same time, if part of the trench 30 separates the MEMS element 10from a part 14 of the IC that is not intended to interact with the MEMSelement 10, it may be advantageous to increase the width of the trench30 in order to limit the magnitude of a parasitic capacitance across thetrench 30, and/or to increase access to the underlying sacrificial layersuch that the MEMS element 10 can be released more quickly.

Design rules may limit the maximum width of the trench 20 and 30, forinstance when a sacrificial layer is to be formed over the MEMS element10 to facilitate the formation of a capping layer, as will be explainedin more detail below. In such a case, the width of the trenches islimited by the design rule to preventingress of the sacrificial materialinto the trenches 20, 30 or the cavity formed underneath the MEMSelement 10.

Hence, it should be understood that many different dimensions for thetrenches 20, 30 may be chosen based on design considerations. However,the IC will typically comprise at least one trench that is chosen to beas narrow as possible to maximize the sensitivity of the MEMS, becausethe trench separates a region of the IC for manipulating or sensing thedisplacement of the MEMS element 10. Such a narrow trench isparticularly prone to filament formation across the trench as previouslyexplained. The present invention provides a set of design rules for thetrench dimensions, or more generally, for the layout of the variousparts of the cavity in which the MEMS element 10 is suspended, to reducethe risk of such filament formation.

An example process flow for manufacturing an IC comprising a MEMS asshown in FIG. 2 is shown in FIG. 3. The various steps of FIG. 3schematically depict the cross-sections across the line A-A in FIG. 2.In step (a), a substrate 100 is provided, which may be any suitable typeof substrate, e.g. an insulating substrate, a silicon substrate, a SiGesubstrate and so on, onto which a first sacrificial material layer 110is deposited or formed, such as an oxide layer, e.g. SiO₂. An activematerial layer 120, e.g. a silicon layer, is deposited or grown over thefirst sacrificial material layer 110. In an embodiment, the combinedstructure comprises a silicon on insulator substrate, which may beprovided as a single structure.

In step (b), a mask 130, e.g. a patterned resist, is formed on theactive material layer 120, which defines the various structures to beformed in the active material layer 120, such as the MEMS element 10 andthe trenches 20 and 30. The mask 130 may be formed in any suitablemanner. Next, the portions of the active material layer 120 that areexposed through the mask 130 are removed, e.g. using a suitable etchrecipe that terminates on the first sacrificial material layer 110, asshown in step (c).

Next, part of the first sacrificial material layer 110 is selectivelyremoved through the patterned active material layer 120, e.g. using anetch recipe that has a high selectivity for the first sacrificialmaterial, as shown in step (d). This releases the MEMS element 10. Aportion of an actuator 12, e.g. a gate electrode, or a sensing elementfor the MEMS element 10 is also shown for the sake of completeness.

A second sacrificial material layer 140 is subsequently deposited orotherwise formed over the resultant structure. A particularly suitablematerial is aluminium, as this can be deposited with high conformality,thus avoiding excessive contamination of the void created by theselective removal of the first sacrificial material layer 110. This isshown in step (e). As previously explained, design rules may be in placeto limit the maximum width of the trenches 20 and 30 to avoid ingress ofthe second sacrificial material.

The method subsequently proceeds to step (f), in which a capping layer150 is deposited or otherwise formed over the second sacrificialmaterial layer 140, such as a silicon nitride layer or a layer ofanother suitable capping material. The capping layer includes one ormore openings, i.e. vent holes, 152 for releasing the second sacrificialmaterial.

Next, the second sacrificial material is selectively removed from the ICas shown in step (g). This is typically achieved using a suitable wetetch recipe having a high selectivity for the second sacrificialmaterial. This step forms the cavity 160 in which the MEMS element 10 issuspended. As will be immediately apparent to the skilled person, thepatterned active material layer 120 typically further comprises one ormore anchoring structures that anchor or suspend the MEMS element 10inside the cavity 160, but these anchoring structures are not explicitlyshown for the sake of brevity.

The cavity 160 is subjected to a sequence of cleaning steps, which willbe explained in more detail later, after which the desired atmosphere isformed in the cavity 160 and the cavity 160 is sealed by plugging theopenings 152 using a suitable sealing material, e.g. a metal such asaluminium, as shown in step (h).

When cleaning the cavity 160, e.g. to remove residues from the one ormore wet etching steps, the cavity 160 is typically subjected to one ormore rinsing cycles, after which the cavity 160 is dried prior to thesealing of the openings 152. Due to the fact that the cavity 160 istypically formed by a number of different sections having differentdimensions, e.g. the voids formed by the removal of the first and secondsacrificial materials and the trenches formed in the active materiallayer 120 to release the MEMS element 10 from the remainder of theactive material layer 120, the drying process exhibits distinct stagesin which different parts of the cavity 160 become dry at differentpoints in time. This is schematically depicted in FIG. 4 for a prior artIC.

The left hand panes of FIG. 4 depict a top view of the active materiallayer 120 including the MEMS element 10, a first region 20 of the cavity160 that defines the gap between the MEMS element 10 and an unreleasedportion of the active material layer 120, which faces a moving edge ofthe MEMS element 10 and for instance may be arranged to interact withthe MEMS element 10, e.g. may define or comprise a gate electrode orsensing electrode 12, and a second region 30 of the cavity 160 thatprovides sufficient clearance between the MEMS element 10 and theportion 12 at the one hand and the remainder of the active materiallayer 120 on the other hand. In FIG. 4, the remainder of the activematerial layer 120 is not designed to interact with the MEMS element 10,such that the trench 30 typically is wider than the trench 20 to provideaccess to the first sacrificial material 110 and provide electricalinsulation as previously explained.

In the top view panes in FIG. 4, the trench 30 is shown in a truncatedform of clarity reasons only. It should be understood that the trench 30typically extends along a substantial part of the MEMS element 10, e.g.the trench 30 may terminate at one of the anchoring regions 16, 18.

The right hand panes of FIG. 4 depict a cross-section (X-section) of theIC along the dashed line in the left hand pane of FIG. 3( a). A liquidbath 300 is connected to the cavity 160 via vent holes 152. Also shownare the lower region 162 of the cavity 160 formed by the removal of thefirst sacrificial material 110 and the upper region 164 of the cavity160 formed by the removal of the second sacrificial material 140, withthe lower region 162 and the upper region 164 being interconnected viathe first region, i.e., actuation gap, 10 and the second region, i.e.,isolation trench, 20 in the active material layer 120.

In FIG. 4 and subsequent figures, the presence of a liquid in the cavity160 or liquid bath 300 is shown in black. The absence of a liquid in thecavity 160 or liquid bath 300 is shown in white.

Step (a) shows the situation at the end of a cleaning cycle immediatelyprior to drying, in which the cavity 160 may have been rinsed with anumber of cleaning or rinsing liquids. At this stage, although the etchresidues may have been sufficiently removed from the cavity 160, theliquid in the cavity 160 will nevertheless still contain a concentrationof unwanted residues, the origin of which may be difficult to control oreven determine. For instance, even in a clean room environment, it maybe practically unfeasible to prevent diffusion of contaminants into therinsing liquid, or indeed the rinsing liquid itself may comprise acertain concentration of contaminants. It may also be difficult toeffectively remove all etch residues from the rinsing liquids as thismay require unfeasibly high dilution. It should be understood that thepresent invention is not concerned with the exact origin of thesecontaminants, as the present invention is not directed to the preventionof such contaminants or residues occurring in the rinsing liquids.

At point (a) in FIG. 4, the residue concentration in the rinsing liquidmay be expressed as C₀, assuming that same concentration is obtainedthroughout the cavity 160 and the liquid bath 300. The exact value C₀will depend on whether this concentration is diffusion-limited, i.e.higher amounts of rinsing liquid will reduce the concentration byfurther dilution, or whether this concentration is supply-limited, i.e.whether a dynamic equilibrium is formed between the source of thecontamination and the rinsing liquid.

The drying process commences by the removal of the liquid bath 300,which is shown in step (b) as the liquid bath 300 becoming dry. In caseof supply-limited contamination, the residue concentration C₁ in theliquid remaining in the cavity 160 will be C₁=C₀. In case ofdiffusion-limited contamination, the residue concentration C₁ in theliquid remaining in the cavity 160 will increase, i.e. C₁>C₀, with C₁being a function of the drying time and temperature, i.e. C₁(t₁, T₁).

Upon further drying, the upper region 164 of the cavity 160 will becomedry first. This reduces the overall volume of the liquid in the cavity160, which therefore increases the contaminant concentration in theremaining liquid, as the contaminants are now ‘locked into’ the cavity160, i.e. the amount of contaminants can now be considered constant,such that from here on changes in contaminant concentration will bediffusion-limited. Consequently, the new contaminant concentration C₂ inthe liquid can be expressed as C₂=C₀·V₁₆₀/(V₁₆₀−V₁₆₄) in case of thesupply-limited model and as C₂=C₁(t₁, T₁)·V₁₆₀/(V₁₆₀−V₁₆₄) in thediffusion-limited model, in which V₁₆₀ and V₁₆₄ are the respectivevolumes of the cavity 160 and the upper portion 164 of the cavity 160.

In step (d), the lower portion 152 is the next portion of the cavity tobecome dry. This is because the liquid that is removed from the trenches20 and 30 in the active material layer 120 is immediately replenished bythe liquid in the lower portion 152 of the cavity 150, at least in partdue to the capillary forces that exist in the relatively narrow trenches20 and 30. Consequently, the new contaminant concentration C₃ in theliquid can be expressed as C₃=C₀·V₁₆₀/(V₁₆₀−V₁₆₂−V₁₆₄) in case of thesupply-limited model and as C₂=C₁(t₁, T₁)·V₁₆₀/(V₁₆₀−V₁₆₂−V₁₆₄) in thediffusion-limited model, in which V₁₆₂ is the volume of the lowerportion 162 of the cavity 160.

In a final step, the cavity regions 20 and 30 in the plane of the activematerial layer 120 will become dry. This is shown in step (e), whichshows the top views of the various stages of this drying process. First,the wide trenches 30 will start to dry, as the larger width of thesetrenches corresponds to a smaller capillary force therein compared tothe capillary forces that exist in the narrow actuation gap or trench20. This is indicated by the void formation in the first and second topviews in the sequence of top views in step (e).

At this point, it is noted that under quasi-static conditions, anyredistribution of the liquid (driven by evaporation) will occur wheredE/dV is at a minimum, i.e. most negative, value, wherein E is the totalsurface energy of the connected liquid and V is the total liquid volume.

For two voids to form simultaneously, this requires that dE/dV becontinuously and simultaneously identical and at a minimum at both voidlocations. This is highly unlikely, and for this reason the dryingprocess of a region of the cavity typically is driven by single voidformation.

After the drying of the wide trenches 30, the new contaminantconcentration C₄ in the liquid can be expressed as C₃=C₀·V₁₆₀N₂₀ in caseof the supply-limited model and as C₂=C₁(t₁, T₁)·V₁₆₀/V₂₀ in thediffusion-limited model, in which V₂₀ is the volume of the actuation gap20 of the cavity 160.

Finally, the cavity region defining the actuation gap or trench 20 willdry, which will lead to the formation of droplets of the rinsing liquidbridging the gap between the MEMS element 10 and the opposite wall ofthe cavity 160. The maximum concentration in the droplet may beexpressed as:

$C_{\max} = {\frac{A_{20}}{{2 \cdot V_{\min}}A_{30}}{C_{0} \cdot \left( {V_{152} + V_{154}} \right)}}$

in which A20 and A30 are the areas of trenches 20 and 30 respectively.For a 200 nm³ droplet, this equates to a 750-fold reduction of a typicalvolume of an actuation gap, such that the contaminant concentration inthis droplet can be expressed as C_(max)=750·C₄, which can quite easilyequate to 4.5*10⁷·C₀ for a typical cavity volume. At suchconcentrations, super-saturation of the droplet is likely to occur,leading to filament formation across the first cavity region 20 by thesolidification of the contaminants in the droplet.

This problem is addressed by the present invention as shown in FIG. 5.The design of the electrical isolation trench 30 is altered to reducethe concentration of contaminants in the first cavity region 20, e.g. anactuation trench or more generally a trench defined by a moving edge ofthe MEMS element 10 on the one side and a wall portion of the cavity onthe other side. More specifically, the electrical isolation trench 30 isdivided into a second cavity region 32 from which the first cavityregion 20 extends and a third cavity region 34 that is separated fromthe second cavity region 32 by a narrow channel 36.

The narrow channel 36 may be located in a part of the trench 30 that islocated between unreleased parts of the active material layer 120, or atleast preferably is not located in a part of the trench 30 adjacent tothe MEMS element 10, as this could increase the parasitic capacitancebetween the MEMS element 10 and its surroundings as well as encouragefilament formation between the MEMS element 10 and its surroundingsacross the narrow channel 36. Consequently, the third cavity region 34of the trench 30 is also located away from the MEMS element 10.

By locating the narrow channel 36 away from the MEMS element 10, e.g. inbetween the unreleased region 12 and a boundary section of the cavity160, filament formation is not necessarily suppressed but is at leastshifted to a part of the MEMS that is insensitive to such filamentformation, i.e. where filament formation has no detrimental impact onthe behaviour of the MEMS element 10, as the filaments are not formed onan edge of the MEMS element 10 that is arranged to be displaced relativeto the opposing cavity wall portion.

In order to suppress filament formation in the first cavity region 20,the second cavity region 32 preferably has at least a section having alarger width W₂ than the third cavity region 34 such that the drying ofthe trenches in the active material layer 120 commences with voidformation in, i.e. drying of, the second cavity region 32, due to thesmallest capillary force in this portion. However, the presence of thenarrow channel 36 acts as a pinch point that prevents the migration ofthe void from the second cavity region 32 into the third cavity region34, such that the third cavity region 34 cannot start drying once thesecond cavity region 34 has dried. To this end, the width of the channel36 preferably does not exceed the width of the first cavity region 20.More preferably, the width of the channel 36 is smaller than the widthof the first cavity region 20.

Instead, a further second cavity region 32 is more likely to form a voidand start drying following the drying of the second cavity region 32,thus leading to the drying sequence as shown in FIG. 5. Consequently,the liquid in the third cavity region 34 becomes separated from theliquid in the first cavity region 20. As the volume of the third cavityregion 34 is significantly larger than the volume of the first cavityregion 20, the bulk of the contaminants will be restricted to the firstcavity regions 34, such that the concentration of contaminants (in thedroplets formed) in the first cavity regions 20 is significantlyreduced, thereby reducing the risk of filament formation in the firstcavity regions 20.

Instead, due to the fact that the bulk of the contaminants is restrictedto the third cavity region 34, filament formation is most likely in thenarrow channels or pinch points 36. For this reason, the volume of thethird cavity region 34 should be maximized whilst ensuring that thewidth of the third cavity region 34 is smaller than the width of thesecond cavity region 32 such that the drying of the trench 30, i.e.,void formation, always commences in the second cavity region 32.

It is noted that the drying sequence shown in FIG. 5 is only one of anumber of possible drying sequences of the trenches 20, 30 in the activematerial layer 120. It is however noted that the drying sequence shownin FIG. 5 is considered the worst case sequence in terms of contaminantlevels in the first cavity region 20, such that other drying sequencesare expected to even further reduce the contaminant levels in this gap,thus further reducing the risk of filament formation.

It is furthermore noted that the section 30 of the cavity 160 includingthe second cavity region 32 and the third cavity region 34 separated bya narrow channel 36 may be formed by simply amending the patterning stepof the active material layer 120 as shown in step (c) of FIG. 2, whichmay be achieved by simply amending the pattern in the mask 130.

It should be understood that the layout shown in FIG. 5 is just one of aplethora of possible layouts that support a drying sequence in which theliquid in the first cavity region 20 becomes fluidly disconnected fromthe liquid in the third cavity region 34. FIG. 6 shows one suchalternative, in which the third cavity region 34 does not extend betweentwo instances of a first cavity region 20, but is disrupted instead by asection 122 of the active material layer 120. It will be apparent thatthe drying sequence as shown in FIG. 5 equally applies to the design inFIG. 6, as the location of the void formation is not affected by thisalteration in the layout of the IC and the active material layer 120 inparticular.

Yet another suitable design is shown in FIG. 7. Here, the first cavityregion 20 is fluidly connected to the second cavity region 32 via aportion of the third cavity region 34, wherein the narrow trenches 36are omitted from the design. Instead, the third cavity region 34 has asection that does not form part of the fluid path between the firstcavity region 20 and the second cavity region 32. The consequence ofthis is demonstrated in the drying sequence shown in FIG. 7. Voidformation is initiated in the second cavity region 32 because of thisregion having a larger width than the third cavity region 34 and thefirst cavity region 20. The drying process expands the void into thethird cavity region 34 to the point where the void separates a firstvolume 34′ and a second volume 34″ from the first cavity region 20. Thecontaminants in these volumes can no longer diffuse into the firstcavity region 20, thereby reducing contaminant levels in the firstcavity region 20 and reducing the risk of filament formation therein.

It will be clear that in order to maximize the reduction of thecontaminant levels in the first cavity region 20, the first volume 34′and the second volume 34″ should be maximized in order to trap as manycontaminants in these volumes as possible. This can be achieved forinstance by minimizing the section of the third cavity region 34 thatforms part of the fluid path from the first cavity region 20 to thesecond cavity region 32. A T-junction design in which the first cavityregion 20 and the second cavity region 32 are aligned for instancecomplies with this objective.

Based on the above non-limiting examples, it should be clear that thepresent invention is based on the insight that a void nucleation site,i.e. the second cavity region 32, can be used to physically separate aliquid volume in the first cavity region 20 from a further andpreferably much larger liquid volume in the third cavity region 34 bythe expansion of the void through the fluid path from the first cavityregion 20 to the second cavity region 32. By ensuring that at least apart of the third cavity region 34 lies outside this fluid path, thevoid expansion will fluidly disconnect this part from the first cavityregion 20, thus preventing the contaminants in the isolated part of thethird cavity region 34 from reaching the first cavity region 20 bydiffusion. To this end, it is preferable that the majority of the volumeof the third cavity region 34 lies outside this fluid path, for instanceas shown in the example embodiment in FIG. 7, and it is more preferablethat the entire third cavity region 34 lies outside this fluid path, forinstance as shown in the example embodiments in FIGS. 5 and 6.

At this point, it is further noted that although FIG. 5 depicts anembodiment in which the arrangement of the second cavity region 32 andthe third cavity region 34 are separated by a narrow channel 36 locatedin the same slice as the active material layer 120, it is equallyfeasible to form the second cavity region 32 and the third cavity region34 separated by the narrow channel 36 out-of-plane, e.g. in which thisarrangement is at least partially formed in the upper portion 164. Suchan embodiment is shown in FIG. 8.

In FIG. 8, the second cavity region 32 coincides with the upper region162 of the cavity 160. In this embodiment, the pinch channel 36 and thethird cavity region 34, i.e. the contaminants collection reservoir, arelocated above the unreleased portion 12 of the active material layer120, such that the pinch channel 36 and the third cavity region 34 arebound by the unreleased portion 12 on the one hand and by the cappinglayer 150 on the other hand. The second cavity region 32 is located atleast in part over the MEMS element 10 and is in direct contact with theone or more vent holes 152 in the capping layer 150.

It is noted that in this embodiment, the width of the second cavityregion 32 is not necessarily larger than the width of the third cavityregion 34 because the second cavity region 32 is in direct contact withthe vent hole(s) 152, such that void formation can only initiate in thesecond cavity region 32. In contrast the third cavity region 34 is incontact with the vent hole(s) 152 via the pinch channel 36 only, whichprevents void formation in the third cavity region 34 until theremainder of the cavity 160 has dried. To this end, the pinch channel 36should have a width that does not exceed the width of the first cavityregion 20 and preferably has a smaller width than the first cavityregion 20. In addition, the pinch channels 36 should have a width orheight that is smaller than the width or height of the second cavityregion 32 and third cavity region 34.

It is noted that FIG. 8 is not drawn to scale, and that it is preferablethat the volume of the third cavity region 34 is kept as large aspossible, e.g. larger than that of the second cavity region 32 tomaximize the amount of contaminants trapped in the third cavity region34 for reasons already explained above. In this embodiment, it ispreferable that drying of the cavity 160 is commenced once a homogeneouscontamination distribution is achieved, such that a substantial amountof the contaminants can be trapped in the second portion 34.

The IC of FIG. 8 can be made using the process steps as shown in FIG. 3,with an additional patterning of the second sacrificial material 140 todefine the pinch channel 36 and the second region 34 prior to thedeposition of the capping layer 150, after which the IC may be finalizedas shown in FIG. 3.

In a further embodiment, shown in FIG. 9( a), the first cavity region,e.g. an actuator gap 20 comprises one or more channels 22 that extendaway from the MEMS element 10. The channels 22 have a width that doesnot exceed the width of the first cavity region 20, and preferably havea width that is smaller than the width of the first cavity region 20 toensure that void formation commences in the actuation first cavityregion 20 rather than in the channels 22. As shown in FIG. 9( b), thetrench 30 of the cavity 160 will dry first (the second cavity region 32and third cavity region 34 are omitted from the trench 30 for the sakeof clarity only), after which void formation will take place in thefirst cavity region 20, as shown in FIG. 9( c). The last portions to drywill be the channels 22, as shown in FIG. 9( d). This means that most ofthe contaminant or residue will accumulate in the channels 22, whichtherefore can be seen to act as a residue sink. This consequently lowersthe contaminant concentration in the first cavity region 20, therebyreducing the risk of filament formation in this region.

The effect of the number of channels 22 on the residue concentration inthe first cavity region 20 has been simulated. The results are shown inFIG. 10, which depicts the concentration ratio between the residue inthe first cavity region 20 and the residue in the channels 22 as afunction of the number of channels and the combined area a_(sink) ofthese channels. The parameter a_(act) defines the area of the firstcavity region 20. This clearly demonstrates that a 10-fold reduction ofthe residue concentration in the first cavity region 20 can be readilyachieved by the provision of such channels 22.

It is not necessary that the first cavity region 20 extends from thewidest sub-section of the second cavity region 32. An example embodimentwhere this is not the case is shown in FIG. 11. Here, the second cavityregion 32 comprises a sub-section 32′ that is arranged in between theMEMS element 10 and an unreleased portion of the active material layer120, with the first cavity region 20 extending from the narrower portion32. The pinch channel 36 separates the third cavity region 34 of thetrench 30 from the second cavity region 32, and is located away from theMEMS element 10 for the same reasons as previously explained. Thesub-section 32 of the second cavity region has a width that is largerthan the width of the third cavity region 34, such that void formationcommences in the sub-section 32 rather than in the third cavity region34.

In summary, the present invention provides a cavity structure in whichreservoirs are provided that direct the filament formation away from thesensitive trenches separating the MEMS element 10 from its surroundings,i.e. the trenches facilitating the displacement of an edge of the MEMSelement 10, thereby reducing the risk that filaments bridging suchsensitive trenches and thereby affecting the dynamic behaviour of theMEMS element 10 are formed. Although the present invention has beendescribed in terms of a capacitive resonator element 10, it should beunderstood that the principles of the present invention may be equallyapplied to any other suitable type of MEMS devices.

It should be noted that the above-mentioned embodiments illustraterather than limit the invention, and that those skilled in the art willbe able to design many alternative embodiments without departing fromthe scope of the appended claims. In the claims, any reference signsplaced between parentheses shall not be construed as limiting the claim.The word “comprising” does not exclude the presence of elements or stepsother than those listed in a claim. The word “a” or “an” preceding anelement does not exclude the presence of a plurality of such elements.The invention can be implemented by means of hardware comprising severaldistinct elements. In the device claim enumerating several means,several of these means can be embodied by one and the same item ofhardware. The mere fact that certain measures are recited in mutuallydifferent dependent claims does not indicate that a combination of thesemeasures cannot be used to advantage.

1. An integrated circuit comprising a MEMS (microelectromechanicalsystem) element in a plane of the integrated circuit, the MEMS elementbeing suspended in a cavity over a substrate, said cavity including: afirst cavity region in said plane spatially separating an edge of theMEMS element from a wall section of the cavity, said edge being arrangedto be displaced relative to the wall section; and a second cavity regionin said plane forming part of a fluid path further including the firstcavity region, said fluid path defining a first volume; and a thirdcavity region in said plane defining a second volume in fluid connectionwith the second cavity region, wherein the maximum width of the secondcavity region is larger than the maximum width of the third cavityregion, the second and third cavity regions having maximum widths thatare larger than the maximum width of the first cavity region, andwherein at least a part of the second volume is excluded from the fluidpath.
 2. The integrated circuit of claim 1, wherein the plane coincideswith a patterned material layer in which the MEMS element is formed. 3.The integrated circuit of claim 1 or 2, wherein the second volume islarger than the first volume.
 4. The integrated circuit of claim 1,wherein the majority of the second volume is excluded from the fluidpath.
 5. The integrated circuit of claim 1, wherein the second volume isexcluded from the fluid path.
 6. The integrated circuit of claim 5,wherein the first cavity region extends from the second cavity region,and wherein the third cavity region is fluidly connected to the secondcavity region via a channel portion having a width not exceeding thewidth of the first cavity region.
 7. The integrated circuit of claim 5,wherein the integrated circuit further comprises a capping layer, andwherein the cavity further comprises a first further region in betweenthe plane and the substrate and a second further region in between theplane and the capping layer.
 8. The integrated circuit of any of claim1, further comprising a plurality of channels in said plane extendingfrom the first cavity region into the wall section, each of saidchannels having a channel width that does not exceed the width of thefirst cavity region.
 9. An integrated circuit comprising: a MEMS(microelectromechanical system) element formed in a material layer overa substrate and being suspended in a cavity; a capping layer over thecavity, said capping layer comprising a plugged opening; wherein thecavity comprises: a first cavity region spatially separating an edge ofthe MEMS element from a wall section of the cavity, said edge beingarranged to be displaced relative to the wall section; a first furtherregion in between the MEMS element and the substrate; and a secondfurther region located in part in between the MEMS element and thecapping layer, said second further region comprising: a first section inbetween the MEMS element and a part of the capping layer including theplugged opening; and a second section in between an unreleased portionof the material layer and a further part of the capping layer notincluding a plugged vent hole, wherein the first section is separatedfrom the second section by a channel portion having a width notexceeding the maximum width of the first cavity region.
 10. Theintegrated circuit of claim 9, wherein the volume of the second sectionis larger than the volume of the first section.
 11. A method ofmanufacturing an integrated circuit comprising a MEMS element suspendedin a cavity, the method comprising: providing a first sacrificialmaterial layer on a substrate; providing an active material layer on thefirst sacrificial material layer; patterning the active material layerto form the MEMS element, a first cavity region spatially separating anedge of the MEMS element from a wall section of the cavity, said edgebeing arranged to be displaced relative to the wall section; a secondcavity region forming part of a fluid path further including the firstcavity region, said fluid path defining a first volume; and a thirdcavity region defining a second volume in fluid connection with thesecond cavity region, wherein the maximum width of the second cavityportion is larger than the maximum width of the third cavity regions,the second and third cavity regions having maximum widths that arelarger than the maximum width of the first cavity region, and wherein atleast a part of the second volume is excluded from the fluid path;forming a part of the cavity by partially removing the first sacrificialmaterial layer through the patterned active material layer using a firstetch recipe; forming a second sacrificial material layer over thepatterned active material layer; forming a capping layer over the secondsacrificial material layer, said capping layer comprises at least oneopening; and completing said cavity by removing the second sacrificialmaterial layer through the at least one opening using a wet etch recipe.12. A method of manufacturing an integrated circuit comprising a MEMSelement suspended in a cavity, the method comprising: providing a firstsacrificial material layer on a substrate; providing an active materiallayer on the first sacrificial material layer; patterning the activematerial layer to form the MEMS element, a first cavity region spatiallyseparating an edge of the MEMS element from a wall section of thecavity, said edge being arranged to be displaced relative to the wallsection; forming a part of the cavity by partially removing the firstsacrificial material layer through the patterned active material layerusing a first etch recipe; forming a second sacrificial material layerover the patterned active material layer; patterning the secondsacrificial material layer to define a first section on the MEMSelement, a second section on an unreleased portion of the material layerand a channel portion connecting the first section to the secondsection, the channel portion having a thickness not exceeding themaximum width of the first cavity region; forming a capping layer overthe second sacrificial material layer, said capping layer comprises atleast one opening over the first section; completing said cavity byremoving the second sacrificial material layer through the at least oneopening using a wet etch recipe.
 13. The method of claim 12, wherein thepatterning step further comprises defining a plurality of channelsextending from the first cavity region into said wall section, each ofsaid channels having a channel width that does not exceed the maximumwidth of the first cavity region.
 14. The method of claim 12, whereinthe second section has a larger volume than the first section.
 15. Themethod of any of claim 12, wherein the active material layer comprises asilicon layer.